1. Field of the Invention
The present invention relates to a sampling offset estimation apparatus and method of an orthogonal frequency division multiplexing (OFDM) system that can be applied to digital television reception systems, and more particularly, to a sampling frequency offset estimation apparatus and method of an OFDM system that can reduce noise power and errors caused by noise power by adding an interpolation between first complex conjugate multiplication and second complex conjugate multiplication of a signal subjected to FFT in an OFDM receiver.
2. Description of the Related Art
In general, an OFDM system includes an OFDM transmitter that transforms data symbols using an inverse fast Fourier transform (IFFT) and transmits the data symbols, and an OFDM receiver that performs a fast Fourier transform (FFT) on a signal from the OFDM transmitter to recover data. Here, the OFDM receiver is shown in FIG. 1.
FIG. 1 is a block diagram illustrating a general OFDM receiver. Referring to FIG. 1, the genera OFDM receiver includes an analog-to-digital (A/D) conversion unit 10 that samples a received signal according to the sampling frequency to convert the received signal into a digital signal, a serial-to-parallel conversion unit 20 that converts the signal from the A/D conversion unit 10 so that serial format is converted into parallel format, a guard interval removal unit 30 that removes a guard interval from the signal from the serial-to-parallel conversion unit 20, an FFT unit 40 that performs an FFT on the signal from the guard interval removal unit 30, a sampling frequency offset estimation unit 50 that estimates sampling frequency offset {circumflex over (η)}m using signals Zm,k from the FFT unit 40, a loop filter 60 that filters a sampling frequency offset estimation value from the sampling frequency offset estimation unit 50, and a local oscillation unit 70 that varies the sampling frequency according to the sampling frequency offset estimation value from the loop filter 60 and supplies the varied sampling frequency to the A/D conversion unit 10.
The A/D conversion unit 10 includes a sampler sampling the received signal according to the sampling frequency and an A/D converter 12 converting the signal sampled by the sampler 11 into a digital signal.
In the above-described OFDM system, sampling in the time domain needs to be performed beforehand in order to demodulate data. A sampling phase offset and a sampling frequency offset that are generated due to sampling errors between the OFDM transmitter and the OFDM receiver cause performance deterioration of the OFDM system. Therefore, the sampling frequency offset needs to be corrected.
FIG. 2 is a view illustrating a frame structure of an OFDM signal. Referring to FIG. 2, a super-frame of an OFDM signal consists of four frames (frame 1 to frame 4). Each of the four frames consists of 68 symbols (symbol 0 to symbol 67). Each of the 68 symbols consists of a guard interval (GI) 21 and a valid data interval 22 having a plurality of cells.
Here, the valid data interval 22 includes 2048 cells (cell0 to cell2047) for 2K mode, and the valid data interval 22 includes 8192 cells (cell0 to cell8191) for 8K mode.
Further, the guide interval 21 is copied to the final portion of the valid data interval 22.
FIG. 3 is a structural view illustrating pilots and data of OFDM symbols. Referring to FIG. 3, an OFDM symbol consists of 6817 subcarriers for 8K mode and 1705 subcarriers for 2K mode.
One symbol includes continuous pilots 31, scattered pilots 32 and subcarriers 33 having data.
In a DVB-T/H system, the continuous pilots 31 and the scattered pilots 32 of the OFDM symbol are modulated by a pseudo-random binary sequence (PRBS) according to Equation 1, and the modulated continuous pilots and scattered pilots are boosted and then transmitted. Here, a polynomial equation used to obtain a PRBS is shown as follows: X11+X2+1.Re{Cm,k}=4/3×2(1/2−ρk),Im{Cm,k}  Equation 1
where m is a symbol number, k is a subcarrier number and ρk is a k-th reference sequence bit corresponding to a k-th subcarrier.
Referring to FIG. 3, the position of the scattered pilots 32 is determined according to Equation 2. The arrangement of the scattered pilots 32 is repeated every four symbols, and each of the scattered pilots is assigned at an interval of 12 subcarriers within one symbol. In 8K mode, 177 continuous pilots 31 are assigned at fixed positions, and in 2K mode, 45 continuous pilots are assigned at fixed positions.k=Kmin+3×(m mod4)+12p where p integer, p≧0, k ε [Kmin;Kmax]  Equation 2
Referring to FIGS. 1 through 3, in the OFDM receiver, on the assumption that another synchronization process other than sampling is completely performed, complex symbols Zm,k in the frequency domain where the sampling frequency offset is generated satisfy the following Equation 3.
                              z                      m            ,            k                          =                                            H                              m                ,                k                                      ·                          a                              m                ,                k                                      ·                                          sin                ⁡                                  (                                      π                    ⁢                                                                                  ⁢                    k                    ⁢                                                                                  ⁢                    η                                    )                                                            N                ⁢                                                                  ⁢                                  sin                  ⁡                                      (                                                                  π                        ⁢                                                                                                  ⁢                        k                        ⁢                                                                                                  ⁢                        η                                            N                                        )                                                                        ·                          exp              ⁡                              (                                  j                  ⁢                                                            2                      ⁢                      π                      ⁢                                                                                          ⁢                      k                                        N                                    ⁢                                      (                                                                  mN                        s                                            +                                              N                        g                                                              )                                    ⁢                  η                                )                                              +                                    ∑                                                i                  =                  0                                ,                                  i                  ≠                  k                                                            N                -                1                                      ⁢                                          H                                  m                  ,                  k                                            ·                              a                                  m                  ,                  k                                            ·                                                sin                  ⁢                                                                          ⁢                                      π                    ⁡                                          (                                              i                        +                                                  k                          ⁡                                                      (                                                          η                              -                              1                                                        )                                                                                              )                                                                                        N                  ⁢                                                                          ⁢                                      sin                    ⁡                                          (                                                                        π                          ⁡                                                      (                                                          i                              +                                                              k                                ⁡                                                                  (                                                                      η                                    -                                    1                                                                    )                                                                                                                      )                                                                          N                                            )                                                                                  ·                              exp                ⁡                                  (                                      j                    ⁢                                                                                  ⁢                    π                    ⁢                                                                                            (                                                      i                            +                                                          k                              ⁡                                                              (                                                                  η                                  -                                  1                                                                )                                                                                                              )                                                ⁢                                                  (                                                      N                            -                            1                                                    )                                                                    N                                                        )                                                              +                      W                          m              ,              k                                                          Equation        ⁢                                  ⁢        3            where η is relative sampling frequency offset, expressed as “η=(T′−T)/T”, T is a rated sample period, T′ is a sampling period of the receiver where offset is generate, Hm,k is channel frequency response (CFR) of a multipath fading channel, and Wm,k is additive white Gaussian noise (AWGN) when mean is zero and variance is σ2, N is the FFT size, “Ng” is the length of a guard interval, and “Ns=N+Ng” is the length of an OFDM symbol.
According to a general sampling frequency offset estimation method in this OFDM system, when continuous pilots are used, frequency offset is estimated using the phase rotation difference between pilots having the same positions in two consecutive OFDM symbols in the frequency domain. An estimation value {circumflex over (η)}m of the sampling frequency offset is obtained according to Equation 4:
                                              ⁢                                                                              η                  ^                                m                            =                                                N                                      2                    ⁢                    π                    ⁢                                                                                  ⁢                                          b                      k                                        ⁢                                                                  N                        s                                            ⁡                                              (                                                                              N                            cp                                                    -                          1                                                )                                                                                            ⁢                                                      ∑                                          k                      =                      0                                                              N                      -                      1                                                        ⁢                                                            tan                                              -                        1                                                              ⁡                                          (                                              R                                                  m                          ,                          k                                                                    )                                                                                            ,                          k              ∈                              S                cp                                              ⁢                                          ⁢                                    R                              m                ,                k                                      =                                                  ⁢                                                            (                                                            z                                                                        m                          +                          1                                                ,                                                  k                          +                                                      b                            k                                                                                                                ·                                          z                                              m                        ,                                                  k                          +                                                      b                            k                                                                                              *                                                        )                                ·                                                      (                                                                  z                                                                              m                            +                            1                                                    ,                          k                                                                    ·                                              z                                                  m                          ,                          k                                                *                                                              )                                    *                                            =                              exp                ⁡                                  [                                                            j                      ⁡                                              (                                                  2                          ⁢                          π                          ⁢                                                                                                          ⁢                                                      b                            k                                                    ⁢                                                                                    N                              s                                                        /                            N                                                                          )                                                              ⁢                    η                                    ]                                                                                        Equation        ⁢                                  ⁢        4            
where Scp is a set of continuous pilots, Ncp is the number of continuous pilots, and bk is spacing between continuous pilots.
In the DVB-T/H system, scattered pilots may also be used for the sampling frequency offset estimation. Since the number of scattered pilots is greater than that of continuous pilots, sampling frequency offset estimation can be more accurately performed. When scattered pilots are used, the estimation value {circumflex over (η)}m of the sampling frequency offset satisfies the following Equation 5:
                                              ⁢                                                                              η                  ^                                m                            =                                                N                                      2                    ⁢                    π                    ⁢                                                                                  ⁢                                                                  bN                        s                                            ⁡                                              (                                                                              N                            sp                                                    -                          1                                                )                                                                                            ⁢                                                      ∑                                          k                      =                      0                                                              N                      -                      1                                                        ⁢                                                            tan                                              -                        1                                                              ⁡                                          (                                              R                                                  m                          ,                          k                                                                    )                                                                                            ,                          k              ∈                              S                sp                                              ⁢                                          ⁢                                    R                              m                ,                k                                      =                                                  ⁢                                                            (                                                                                    z                                                                              m                            +                            1                                                    ,                                                      k                            +                            b                            +                            3                                                                                              ·                                              z                                                  m                          ,                                                      k                            +                            b                                                                          *                                                                                                   )                                ·                                                      (                                                                                            z                                                                                    m                              +                              1                                                        ,                                                          k                              +                              3                                                                                                      ·                                                  z                                                      m                            ,                            k                                                    *                                                                                                             )                                    *                                            =                              exp                ⁡                                  [                                                            j                      ⁡                                              (                                                  2                          ⁢                          π                          ⁢                                                                                                          ⁢                                                                                    bN                              s                                                        /                            N                                                                          )                                                              ⁢                    η                                    ]                                                                                        Equation        ⁢                                  ⁢        5            where Ssp is a set of scattered pilots, Nsp is the number of scattered pilots, and b is 12, which is spacing between scattered pilots within one symbol.
However, the general sampling frequency offset estimation method results in signal to noise ratio (SNR) loss due to increases in noise and inter-carrier interference (ICI) components when performing complex conjugate multiplication between adjacent symbols. Furthermore, the channel frequency response (CFR) may not be completely removed by complex conjugate multiplication at high Doppler frequency.
In addition, since these remaining CFR components are multiplied by signal components, they cause significant errors in the estimation. As a result, the result of complex conjugate multiplication performed twice satisfies the following Equation 6:Zm,k=Hm,k·Sm,k+Hm,k+ICIm,k+Wm,k=Hm,k·Sm,k+Vm,k(Zm+1,k+dsp·Zm,k+dsp*)·(Zm+1,k·Zm,k*)*=Ĥm,k·Ŝm,k+{circumflex over (V)}m,k  Equation 6where Ĥm,k is obtained by raising the CFR to the fourth power, {circumflex over (V)}m,k is undesirable components formed by combining CFR and pilot signals, ICI components and AWGN components. These components need to be removed.
As described above, referring to Equation 6, the sampling frequency offset estimation method according to the related art causes deterioration in estimation performance at low SNR and high Doppler frequency.